It's so simple – but the effect is immense. The four bells that toll through the transition scenes of Parsifal, Wagner's great final masterpiece, have an earth-shattering impact.

Parsifal twice journeys to the Grail Hall, in Acts I and III, and each time entails a majestic (and lengthy) scene change. The purely orchestral music is derived from a simple four-note theme, and towards the climax of each passage we hear the pattern played on four bells. The air hums with their complex overtones – bells sound not one note but a whole chord behind the main pitch – and at the height of their tremendous crescendos they almost make the building shake. Their many-note quality makes bells the perfect instrument to reflect the same-yet-different nature of the two transitions: in Act I dignified and heroic; in Act III, as the Grail community disintegrates, desolate and horrifying.

What were the actual sounds Wagner had in mind? Bells at the low pitch he required were hardly conventional orchestral instruments – for good reason, given the lowest pitch bell would weigh more than twenty-six tonnes and have a diameter of around eight metres. Tubular bells or bell plates, popular orchestral bell substitutes, would not cut it. Wagner needed something otherworldly – but also something that could fit in the pit.

There’s no easy solution. Wagner first thought to use tamtams, but after procuring some from London (apparently the nearest tamtam centre) he decided that alone they weren’t enough. He then commissioned a special piano-like instrument from his friend Eduard Steingraeber, with a piano’s metal frame and 24 strings all tuned to four notes, and a keyboard with just four (massive) keys.

For the premiere the Steingraeber was played alongside tamtams, gongs and a tuba, but the result evidently didn’t match the sounds of Wagner’s imagination. During rehearsals his wife Cosima wrote in her diary ‘The orchestra…breaks into hearty applause after the transition scene, which does R. good, though he has many difficulties to contend with: the bells are not right’.

A few years after the 1882 premiere - and Wagner's death the following year - Cosima supervised the construction of four huge metal barrels. These remained in place at Bayreuth for several decades. Other opera houses, once the 30-year embargo on performances outside Bayreuth had expired, had to find their own solutions, usually some combination of tubular bells and gongs.

The development of synthesizers in the early 20th century generated a new set of sounds, though it was some time before they made it to the Grail Hall. The Trautonium, invented by engineer Friedrich Trautwein in 1929, was used by composers throughout the 1930s, particularly Paul Hindemith who worked in collaboration with Oskar Sala, the instrument’s main proponent. It wasn’t until after Richard Strauss had used it to re-create bell sounds for his Japanische Festmusik (1940) that the Trautonium’s Parsifal potential was realized, with Sala taking over bells duty at Bayreuth from 1952.

Synthesizers are now the most widely used bells, and combined with a PA sound system they can have the requisite earthquake-like effect. They’re not without their perils, however, and if misjudged can sound less like bells of solemn majesty and more like an ice-cream van.

Synthesizers have been used at The Royal Opera for the previous two productions – but for Stephen Langridge’s new production Music Director Antonio Pappano has introduced a combination of live instruments, returning to Wagner’s original solution but now supported by modern technology. Tubular bells and bell plates will be played together with a close-miked piano, and the combined sound will be relayed through the Royal Opera House’s sound system. Judging from the rehearsals, it’s going to be thrilling.

N. B.: The first electro-acoustic Grail Bells were performed by the German inventor Jorg Mager in 1931. See Peter Donhauser's book "Elektrische Klangmaschinen: Die Pionierzeit in Deutschland und Osterreich" (Vienna: Bohlau, 2007), pp. 200-203.